Detecting Explosives with Gamma Rays A needle in a haystack Most plastic explosives, such as Semtex, military C4, and Detasheet, are dense materials that contain large amounts..., and their nitrogen or chlorine density. Total density alone is not sufficient to separate contraband from common materials or materials deliberately used for concealment; concrete, salt... nitrogen densities similar to those of explosives. But a high probability of detection and a low false-alarm rate can be achieved by examining both total density and nitrogen density...